The present invention pertains to combustors for use in gas turbine engines, and more particularly for cooling systems therefor.
Combustors for use in gas turbine engines, particularly of the variety used to power aircraft, must be lightweight, stiff and reliable, as well as efficiently cooled. The combustor must be light enough in weight to permit the aircraft variety of these engines to successfully fly. The combustor wall, however, must also be substantial enough to withstand aerodynamic forces (resulting from the rapid expansion of oxidizing fuel within the combustion zone) as well as buckling forces associated with pressure drops created across the wall by a surrounding cooling fluid.
These combined design characteristics require that the combustor be formed of a thin and lightweight but strong material fabricated in a manner which provides substantial rigidity in both the radial and circumferential directions. One form of combustor includes an inner wall formed of a plurality of telescoping, overlapping annular hoops, where each hoop engages the adjacent upstream hoop in a substantially circular joint in order to define the combustion zone. Combustor walls of this variety have performed successfully in the past. However, the requirements of increased life and reliability have defined a need for combustor walls having increased strength and stiffness. Strengthening has been achieved by the addition of encircling "Z" rings, or by increasing combustor wall cross-sectional thickness. Each of these techniques has inherent disadvantages stemming from the weight factors involved. The present invention provides a reliable solution to this problem by substantially increasing combustor strength and stiffness without the necessity for added overall materials expenditure so that increased weight need not occur.
A contemporaneous problem has been that ever-increasing, nominal temperatures within the combustion zone result in a requirement for increasing the cooling of associated combustors. (The quantity of cooling fluid applied to various portions of gas turbine engines is inversely proportional to the overall engine efficiency; hence, methods for reducing quantities of cooling fluid used while increasing cooling efficiency are particularly important.)
Combustor wall cooling must counteract the heat transferred to the wall by convection and radiation from the hot combustion products in the combustion zone. In the past, convective heating of the liner has been reduced by maintaining a low boundary temperature through the addition of a film of cooling fluid to the heated side of the wall. For this purpose, combustor walls surrounded by cooling fluid plena have been supplied with apertures at appropriate positions for providing communication between the heated wall side and the cooling fluid plenum. The apertures have been formed in a manner to facilitate the formation of a cooling fluid film on the heated wall side.
To supplement the film cooling of combustor walls, the circumscribing cooling fluid plena are arranged so that the fluid therein removes heat from the wall by means of convective heat transfer. This variety of heat transfer is a function of the velocity of cooling fluid which engages the wall. Hence, it is desirable that the fluid velocity within a plenum which fluid engages the wall be relatively large. Unfortunately, high plenum fluid velocities result in large pressure losses creating overall inefficiencies. Furthermore, high fluid velocity within the plenum also affects detrimentally the uniformity of temperature profiles within a combustion zone adversely affecting combustion efficiency. For these and other reasons, it has been found unsatisfactory to provide large cooling fluid velocities throughout the plenum.
The present invention overcomes the foregoing limitations by providing combustor wall stiffening members which increase combustor rigidity for the purpose of increasing life and reliability, and which contemporaneously perform a valuable cooling function by means of directing cooling fluid onto the combustor wall in a plurality of high velocity streams for the purpose of increasing convective heat transfer therefrom. The overall effect of this latter characteristic is increased convective cooling efficiency of the cooling fluid with respect to the combustor without the necessity for increasing overall fluid velocity within the plenum.